benchmark: add plot_csv R graphing script

This commit adds a graphing script (in R) for graphing the CSV output
of a benchmark. It can be run like this:

```
$ OUTPUT_FORMAT=csv iojs benchmark/http/client-request-body.js >
data.csv
$ ./benchmark/plot_csv.R data.csv graph.png bytes type
```

This will graph the output to `graph.png`, using the output's `bytes`
value as X and the result value for each as Y. Output will be grouped
by `type`.

Running as the example yields a beautiful graph like this:
http://pbrd.co/1vBhUfy.

PR-URL: #777
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

benchmark/plot_csv.R

@@ -0,0 +1,38 @@
+#!/usr/bin/env Rscript
+
+# To use this to graph some benchmarks, install R (http://www.r-project.org/)
+# and ggplot (http://ggplot2.org/).
+#
+# Once installed, you can generate some CSV output with a command like this:
+#
+#     $ OUTPUT_FORMAT=csv iojs benchmark/http/client-request-body.js > data.csv
+#     $ ./benchmark/plot_csv.R data.csv data.png bytes type
+#
+# Where the 3rd argument to this script is the graph's X coordinate, the 4th is
+# how the output is grouped, and the Y coordinate defaults to result.
+
+library(methods)
+library(ggplot2)
+
+# get info from arguments
+args <- commandArgs(TRUE)
+
+csvFilename <- args[1]
+graphFilename <- args[2]
+
+xCoordinate <- args[3]
+groupBy <- args[4]
+
+# read data
+data <- read.csv(file = csvFilename, head = TRUE)
+
+# plot and save
+plot <- ggplot(data = data, aes_string(x = xCoordinate, y = 'result', col = groupBy)) +
+        geom_point(size = 5) +
+        ggtitle(data$filename)
+
+png(filename = graphFilename, width = 560, height = 480, units = 'px')
+print(plot)
+graphics.off()
+
+cat(paste('Saved to', graphFilename, '\n'))